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A new methodology of defining the relationship between SOC (state of charge) and OCV (open circuit voltage) relationship of the Li-ion battery pack used on HEVs (hybrid electric vehicles) which is independent of the battery condition was proposed. This methodology could avoid the problems resulting from the defects that the conventional SOC-OCV relationship differs between batteries and different working conditions. Based on the new definition, a state and parameter estimator of the Li-ion battery pack based on the Sage-Husa adaptive Kalman filter was proposed. This estimator recruited an equivalent circuit model to describe the dynamic characteristics of the battery pack. The estimator could estimate the SOC, the battery actual capacity and the inner resistance on-board. The implementation of the estimator on a FPGA platform was also introduced. Testing results show that the new definition and the estimator work very well in any specific working condition.